The field of the invention is nuclear magnetic resonance imaging (MRI) methods and systems. More particularly, the invention relates to characterization of various timing delays in an off-axis MRI system.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments, commonly called spins, in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated, and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
A number of imaging techniques use the spin warp method, sometimes referred to as the Fourier transform (FT) method, in which one or two magnetic field gradients phase encode spatial information in the direction of the gradient. In a two-dimensional implementation (2DFT), for example, spatial information is encoded in one direction by applying a phase encoding gradient along one gradient direction, and then a gradient echo or a spin-echo signal is acquired in the presence of a readout magnetic gradient in a direction orthogonal to the phase encoding gradient. In a typical 2DFT scan, the magnitude of the phase encoding gradient pulse is incremented in the sequence of views that are acquired and Fourier space, or “k-space” is sampled in a Cartesian grid. Most scans currently performed on MRI systems employ such 2DFT or 3DFT techniques.
There are a number of MR imaging techniques which do not use the Fourier transform method of sampling k-space in a Cartesian grid. These include spiral techniques such as that described in U.S. Pat. Nos. 6,215,305 and 6,404,194; projection reconstruction, or radial, techniques such as that described in U.S. Pat. No. 6,794,867; and shell k-space sampling techniques such as that described in U.S. Pat. No. 5,532,595. A common element of these non-Cartesian sampling techniques is that the imaging gradient field changes strength and is time-varying during the read-out of the NMR signal.
Non-Cartesian imaging techniques have several benefits in accelerating magnetic resonance imaging. However, these techniques are more sensitive to system instabilities caused by eddy currents and hardware delays that vary from MRI system to system. While forms of these faster imaging methods are available on clinical platforms, they are generally considered to create artifacts not seen in conventional Cartesian imaging. However, they are used heavily because their speed allows them to capture physiological processes not possible with Cartesian imaging.
One clinical application that is particularly problematic for non-Cartesian imaging techniques is off axis imaging. Imaging off axis or off isocenter in MRI is often necessary because the anatomy of interest cannot be placed at the center of the magnet. Common situations include the knee, shoulder, and heart. Off axis imaging using a Cartesian pulse sequence is easily managed by introducing a constant frequency shift, or equivalent linear phase shift in the received NMR signal which effectively shifts the center of the reconstructed image away from the system isocenter. This is commonly done by modifying the phase of the reference signal used to demodulate the received NMR signals. In Cartesian imaging, this is done by offsetting the frequency of the reference signal for imaging offsets along the readout gradient direction or creating a linear shifting of the phase of the acquired k-space data in the phase-encoding gradient direction. In the readout direction, the required phase shifts are linearly proportional to the image offset along the readout gradient axis and the strength of the readout gradient. When non-Cartesian pulse sequences are used, however, this strategy becomes much more difficult because the time-varying gradients can be considered to be changing the direction and the strength of the readout gradient. The phase shift is no longer simply linear and must be changed in real time as the changing gradient waveforms are played out during the NMR signal acquisition.